How to Correctly Assign a Value to an Int Pointer in Go?

Setting an Int Pointer to an Int Value in Go

In Go, a common task is assigning a value to an int pointer. However, a recently discovered error in a code snippet sheds light on a potential pitfall:

package main

import (
    "fmt"
    "reflect"
)

func main() {
    var guess *int
    fmt.Println(reflect.TypeOf(guess))
    *guess = 12345
    fmt.Println(guess)
}

When executing this code, the following error occurs:

Type: *int
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x483c7d]

goroutine 1 [running]:
main.main()
    /home/aaron/projects/gopath/src/github.com/AaronNBrock/go-guess/main.go:16 +0x9d
exit status 2

The root cause lies in the fact that the pointer guess is initially set to nil. Dereferencing a nil pointer throws a runtime error, as seen in the error message. The solution lies in allocating memory for the pointer before attempting to assign a value to it:

var guess *int
guess = new(int)
*guess = 12345

This modification ensures that guess points to a valid memory location, allowing us to set and retrieve its value.

Alternatively, using a short variable declaration can simplify the code:

guess := new(int)
*guess = 12345

Another approach is to assign the address of an existing variable to the pointer:

value := 12345 // will be of type int
guess := &value

This solution points guess to an existing variable, rather than allocating new memory. Changing the pointed value also modifies the value of the original variable.

Finally, keep in mind that pointing a pointer to a location and assigning a value to it are distinct operations. The former allocates memory or sets the pointer to an existing variable, while the latter modifies the value stored at the pointed location.

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